Connection arrangement for optical communication systems

ABSTRACT

A casing for a communication apparatus having at least one electrical lead for carrying RF signals within the apparatus is provided. The casing includes at least one electrically non-conductive electromagnetic absorber body adapted to at least partly cover the at least one electrical lead.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser.No. 11/332,041 filed on Jan. 13, 2006, now pending, which is adivisional application of U.S. application Ser. No. 10/809,298, filedMar. 25, 2004, now pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to optical communication systems such asfiber optic communication systems.

2. Description of the Related Art

Transmission bit-rates of 10 Gb/s are now current in optical datacommunication systems and higher bit-rates are expected to becomecurrent in the future.

At these operational speeds, using metallic leads to carry electricalsignals between optical and electrical subassemblies may appreciablyaffect radio-frequency (RF) performance. This is particularly true infiber optic transceivers where large volumes of data are aggregated toproduce a very high speed serial data stream, which is then used todrive a light source, e.g. a laser.

Additionally, at very high operational speeds, low cost and/or smalldimension requirements imposed on apparatus give rise to significantproblems in terms of electromagnetic immunity (EMI), electromagneticcompatibility (EMC), RF management and signal integrity.

At 10 Gb/s and higher, the existing solutions for connecting the opticalsubassemblies (OSAs) and the electrical subassembly (ESA) in a fiberoptic transceiver primarily aim at minimizing the interconnection loss.Exemplary of such prior art solutions is the use of a flexible printedcircuit board (PCB). This is oftentimes referred to as a “flex”.

Using a flex generally improves RF performance of the interconnection.However, implementing this solution is time consuming in view of theneed of attaching the flex to the ESA and the OSA. Moreover, the flex isnot self-protected from crosstalk (X-Talk) and is not exempt from EMCproblems.

SUMMARY OF THE INVENTION

A basic object of the invention is to provide an improved solutiondispensing with the problems inherent in such prior art arrangement asthe use of a “flex” discussed in the foregoing.

A specific object of the invention is to provide an improved solutiondispensing with crosstalk developed between transmitter and receiver inan optical communication environment (e.g. in a fiber optictransceiver). Essentially, crosstalk is due to the electromagnetic fieldirradiated from the transmitter (receiver) and picked up by the receiver(transmitter).

Another object of the invention is to provide an arrangement improved interms of RF-performance of the OSA-to-ESA interconnections: these beingusually comprised of a so-called lead frames in air causes theseinterconnections to exhibit an undesirably high impedance.

Still another object of the invention is to provide an arrangementimproved in terms of electromagnetic immunity (EMI) of the RF-paths.

A still further object of the invention is to provide an arrangementimproved in terms of electromagnetic compatibility (EMC) of e.g. atransceiver.

An additional object of the invention is to provide an arrangement thatjointly achieves all the objects considered in the foregoing.

A preferred embodiment of the invention is thus based on the use of alead frame in conjunction with electromagnetic absorber material.Electrical connection of the OSAs and the ESA is ensured by means ofmetallic leads. These leads are easy to produce while simultaneouslyeasy and fast to solder to the ESA.

As indicated, crosstalk and other problems related to EMC and EMI areprimarily related to emissions of electromagnetic fields from the leadsand/or to electromagnetic fields picked up by the leads. Morespecifically, crosstalk is due to the electromagnetic field irradiatedfrom the transmitter (receiver) and picked up by the receiver(transmitter). The influence of these fields is drastically reduced bythe absorber material. Using the absorber material on both the receiver(RX) and the transmitter (TX) side will drastically reduceelectromagnetic emissions. Due to the absorbing properties of thematerial, these electromagnetic emissions are converted into thermalpower, which is easily dissipated.

Additionally, the absorber material reduces the discontinuity in theelectrical path generated when leads in air are used as well as therelative difference of characteristic impedance of the lead zone. Theabsorber material has a dielectric constant higher than air and thuscloser to the dielectric constant of the PCB and the feed throughs ofthe OSAs.

RF-performance of the interconnection will be improved using theabsorber, because the dielectric constant of the material is highlybeneficial in reaching the desired characteristic impedance. Without anabsorber material, a frame comprised of leads in air will typically havean impedance higher than the desired impedance. At least partlyembedding the leads in an absorber material reduces the impedance andthus leads to improved performance.

From the viewpoint of EMI/EMC, the absorber material represents a way toincrease immunity and compatibility in apparatus such as a transceiver.The electromagnetic emissions otherwise likely to be picked up byequipment around the transceiver will be shielded by the externalprotections of the transceiver and eventually absorbed internally to themodule. Additionally, leakage of electromagnetic fields from thetransceiver towards the surrounding environment will be drasticallyreduced because a large portion of them is absorbed internally to themodule.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described, by way of example only, byreferring to he annexed figures of drawing, wherein:

FIG. 1 is a schematic perspective view of a transceiver for fiber opticcommunications,

FIG. 2 is a schematic perspective view of the transceiver shown in FIG.1 showing the application of absorber elements as described herein, and

FIG. 3 shows other parts of a transceiver adapted to be provided withabsorber elements as described herein.

DETAILED DESCRIPTION OF THE INVENTION

As used herein “electromagnetic absorber material” will indicate anymaterial exhibiting the capability of absorbing electromagneticfields/waves. More specifically, electromagnetic fields/waves in thetypical ranges of interest for fiber optic communications (i.e. 30 MHzto 20 GHz) will be primarily considered. More to the point, electricallynon-conductive (i.e. insulating) absorber materials will be considered.

Such materials are currently available for use as free space absorbers,cavity resonance absorbers or load absorbers in microwave products.Typical formulations include magnetically loaded, flexible silicone orurethane sheets. Alternative arrangements include variations in theloading material, such as e.g. iron loaded, ferrite loaded ordielectrically loaded materials (which may exhibit resonant properties)and/or variations in the sheet structure, such as e.g. multilayered,carbon impregnated polyurethane foam sheets, open cell foam sheets withcontrolled conductivity gradient, vinyl plastic or silicone rubbersheets.

Absorber material particularly suitable for use in the invention isavailable as ECCOSORB® FGM-40 from EMERSON & CUMING MICROWAVE PRODUCTS,INC. of Randolph, Mass.

FIG. 1 is a prospective view of a transmitter/receiver (transceiver) forfiber optic communications.

The arrangement shown is essentially comprised of a casing or enclosure10 where optical subassemblies (OSAs) 12 and 14 are provided on thetransmitter side and the receiver side of the transceiver, respectively.

Reference numeral 18 designates a so-called electrical subassembly (ESA)onto which the OSAs 12 and 14 are mounted.

Reference numerals 20 and 22 designate optical couplers ensuring opticalcoupling of the OSAs 12 and 14 with transmitter and receiver fibers (nonshown) associated with the transceiver.

Electrical connection between the OSAs 12 and 14 and the ESA 18 isensured by so-called lead frames 24, 26. Each frame is comprised of aplurality of metallic leads adapted to carry electrical signals.

The arrangement shown in FIG. 1 is conventional in the art andessentially corresponds to the transceiver designated Espresso/XFP™commercially available with the assignee company.

Those of skill in the art will also appreciate that reference to thisbasic arrangement is of purely exemplary nature. The improvedarrangement described herein may be used advantageously in connectionwith basic arrangements substantially different from the one shown inFIG. 1. Just by way of example, reference can be made to arrangementsincluding only one OSA connected with an associated ESA thus producing afiber optic communication system comprised either of a transmitter or areceiver.

FIG. 2 reproduces the same basic arrangement of FIG. 1 where the casing10 has been shown in shadow lines only for the sake of ease and clarityof representation. In FIG. 2, parts/components identical or equivalentto those already described in connection with FIG. 1 are designated bythe same reference numerals.

In FIG. 2, references numerals 28 and 30 designate two elementscomprised of an electrically non-conductive electromagnetic absorbermaterial as defined in the introductory portion of this detaileddescription.

In the exemplary embodiment shown, the elements 28 to 30 are in the formof plate-like elements cut out of a sheet of absorber material having athickness of e.g 1 millimeter. Reference to that specific thickness isof exemplary nature of course.

The dimensions (plan area) of the elements 28 and 30 will be generallyconformed to the size of the lead frames onto which the elements 28 and30 are located.

In typical arrangements, the lead frame 24 interposed between the ESA 18and the transmitter OSA will be generally larger (essentially, willinclude a larger number of leads) then the lead frame 26 associated withthe receiver OSA 14. Accordingly the element 28 will be generally largerthen the element 30, in order to ensure that the leads in the leadframes 24 and 26 are properly covered by the absorber materialcomprising the elements 28 and 30.

As result of being interposed between the absorber elements 28 and 30and the underlying printed circuit board 18 (essentially comprised of adielectric material), the leads included in the lead frames 24 and 26will be at least partly, covered by the absorber elements 28, and 30.

Experiments carried out by the inventor have shown that—in a thoroughlysurprising and unexpected manner—such covering effect of the leads bythe absorber elements 28 and 30 results in improved performance in termsof immunity to crosstalk, RF management, electromagnetic immunity,electromagnetic compatibility and resulting signal integrity.

Even without wishing to be bound to any specific field in that respect,the inventor believes that such unexpected effect is due to thesimultaneous presence, and synergic effect within the electricallynon-conductive electromagnetic absorber materials considered, ofcomponents (e.g. magnetic, iron, ferrite or dielectric loading) adaptedto ensure electromagnetic immunity and electromagnetic compatibility dueto the absorbing action in respect of electromagnetic fields/waves—andthe isolating matrix (silicon, urethane, vinyl plastic or silicon) intowhich such loading material is dispersed. Such matrix presumably acts asa sort of “sheath” that appreciably increases performance in terms of RFmanagement and signal integrity due to the higher dielectric constant ofsuch material in comparison with air.

The resulting sheating effect effectively reduces the discontinuity inthe electrical path in comparison with the situation arising when leadsin air are used, thus notional dispensing with the relative differenceof characteristic impedance in the lead zone.

It will of course be appreciated that using the same material for theelements 28 and 30 is a preferred solution for the sake of simplicity,but is not a mandatory requirement.

Additionally, it will be appreciated that “covering” of the lead framesby the absorber material does in no way require direct exposure/contactof the absorber material to the metallic leads. As used herein,covering/coverage encompasses arrangements where other materials arearranged between the leads and the absorber material, while preservingthe co-extensive nature of the metallic lead(s) and the absorbermaterial.

As shown in FIG. 3, use of absorber materials as disclosed in theforegoing can be easily extended to other areas/portions of a fiberoptic system.

FIG. 3 is essentially a top plan view of the arrangement shown in FIG. 1where only the optical couplers 20 and 28 were left in place in order tobetter visualize the portion of the casing 10 underlying the areas wherethe OSAs 12 and 14 and the ESA 18 are located.

In FIG. 3, reference numerals 32 and 34 indicate two elementsessentially comprised of a sheet-like, electrically non-conductiveelectromagnetic absorber material arranged in order to cover the areasof the casing 10 where the OSAs 12 and 14 and the ESA 18 are located.

Adoption of the arrangement shown in FIG. 3 (preferably in combinationwith the presence of the elements 28 and 30) further emphasizes thebeneficial effects of the arrangement described herein in terms ofelectromagnetic immunity, electromagnetic compatibility, RF management,signal integrity and crosstalk sensitivity. Using the absorber elements28 and 30 of FIG. 2 together with the absorber elements 32 and 34 ofFIG. 3 leads to both lead frames 24 and 26 being covered on both sizesin fact sandwiched between electrically non-conductive electromagneticabsorber material.

Although the invention has been described with a certain degree ofparticularity, it is understood that the present description has beenmade only by way of example and that numerous changes in the details ofconstruction and the combination and arrangement of parts may beresorted to without departing from the spirit and scope of theinvention.

1. A casing for a communication apparatus having at least one electricallead for carrying RF signals within the apparatus, the casingcomprising: at least one electrically non-conductive electromagneticabsorber body adapted to at least partly cover the at least oneelectrical lead.
 2. The casing of claim 1, further comprising adielectric support board for the at least one electrical lead, whereinsaid at least one electrical lead is adapted to be arranged between saidat least one electrically non-conductive electromagnetic absorber bodyand said dielectric support board.
 3. The casing of claim 1, whereinsaid at least one electrically non-conductive electromagnetic absorberbody comprises a material selected from the group consisting of amagnetically loaded material, an iron loaded material, a ferrite loadedmaterial, and a dielectrically loaded material.
 4. The casing of claim1, wherein said at least one electrically non-conductive electromagneticabsorber body comprises a material selected from the group consisting ofsilicon, urethane, vinyl plastic, and silicon rubber.
 5. The casing ofclaim 1, wherein said at least one electrically non-conductiveelectromagnetic absorber body is in the form of a sheet material.